How does The Three Dragons unique work?

Haven't played Path of Exile for over 2 years, and started playing again. I wan't to try out the Three Dragons helmet, but I'm unsure about one thing about this helmet.

http://pathofexile.gamepedia.com/The_Three_Dragons This is the helm in question.

If I want to use Arc, so I can freeze my enemies with lightning damage. Do I want increased shock duration or increased freeze duration to increase the freeze duration?

Freeze chance/duration still applies to freezes. Get that.

Ok, thanks!